The invention relates to an arrangement for feeding in high frequency (HF) current for rotatable tubular cathodes in a vacuum chamber of a plasma coating system as well as a high frequency current source and, located inside said tubular cathode, a stationary magnet arrangement that extends along said tubular cathode, in order to fulfill the objective of generating a magnetic field.
Rotatable tubular cathodes usually consist of a support tube and a target shell, which surrounds said support tube, and are used in plasma coating systems for coating predominantly substrates having a large surface area in a vacuum chamber. During the coating process in the vacuum chamber the substrates, such as plates, sheets or wide metal strips or wide plastic strips or films, pass at a specified distance beneath the tubular cathode. Hence, the length of such tubular cathodes can easily amount to several meters. The HF current that is required to ignite a plasma between the tubular cathode and an anode (substrate) is fed to the tubular cathode over commutator systems with carbon brushes or other sliding contacts and, in particular, outside or inside the vacuum chamber. Furthermore, a stationary magnet arrangement, which extends over the entire length of the tubular cathode, is also located in the tubular cathode and, thus, does not rotate with the tubular cathode.
Working on the aforesaid, the DE 20 2002 005 011 U1 shows such a current infeed for a tubular cathode outside the vacuum chamber, wherein said current infeed extends into the vacuum chamber by means of a vacuum tight rotary feedthrough. The tubular cathode is mounted only on one side and, in particular, on the side of the rotary feedthrough. The disadvantage here is that in the course of feeding in the HF current the vacuum feedthrough is subjected, in particular, to thermal loading due to the eddy currents, which are generated, and, as a result, can begin to leak over time.
Moreover, longer tubular cathodes have to be mounted or rather supported at both ends (DE 10 2006 017 455 A1), in order to guarantee a uniform distance from the substrate over the entire length of said tubular cathodes.
US 2002 0189 939 A1 discloses an infeed of alternating current for tubular cathodes by way of carbon brushes, wherein this alternating current infeed is located inside the vacuum chamber. However, the problem here is that the abrasive wear resulting from the carbon brushes can contaminate the vacuum, so that the vacuum chamber has to be cleaned on a regular basis or even during the sputtering process.
However, both embodiments have the problem that when an HF current is fed in, the current propagates primarily by means of surface conduction to the surface of the tubular cathode, but at a current density that is locally quite different. Thus, the current density in the vicinity of the infeed point is particularly high, whereas at the other end of the tubular cathode a lower current density is observed. The result is a non-homogeneous sputter removal.
In addition and beyond the above described problem, commutator systems with carbon brushes are suitable only to a limited extent for the implementation of an infeed of high frequency current.
At this point the object of the present invention is to provide an arrangement that is designed for feeding in a HF current for tubular cathodes and that enables a uniform infeed of the HF current, so that a particularly homogeneous sputter removal from the tubular cathode is guaranteed.